CREATED:对象创建时间

11gR2数据库对象创建在2011年9月17号，不是很懂，oracle的某些时间设定是非常有趣的。

LAST_DDL_TIME：在对象上执行最后一次ddl的时间,也包括授权时间。

TIMESTAMP：Timestamp for the specification of the object (character data)，对象说明（字符型数据）的创建时间，立马想到comment

SQL> select created,last_ddl_time,timestamp from dba_objects where object_name='TAB1';

CREATED     LAST_DDL_TIME TIMESTAMP

------------------- ------------------- -------------------

2017-06-19 15:57:16 2017-06-19 15:57:34 2017-06-19:15:57:16

SQL> comment on table smith.tab1 is 'for test';

Comment created.

SQL> select created,last_ddl_time,timestamp from dba_objects where object_name='TAB1';

CREATED     LAST_DDL_TIME TIMESTAMP

------------------- ------------------- -------------------

2017-06-19 15:57:16 2017-06-19 15:57:34 2017-06-19:15:57:16

期待的timestamp并没有发生改变。这里的specification很难理解，我们来试试其他的方法。

SQL> create view smith.view1 as select * from smith.tab1;

View created.

SQL> select created,last_ddl_time,timestamp from dba_objects where object_name='VIEW1';

CREATED     LAST_DDL_TIME TIMESTAMP

------------------- ------------------- -------------------

2017-06-19 23:13:18 2017-06-19 23:13:18 2017-06-19:23:13:18

SQL> create or replace view smith.view1 as select * from smith.tab1;

View created.

SQL> select created,last_ddl_time,timestamp from dba_objects where object_name='VIEW1';

CREATED     LAST_DDL_TIME TIMESTAMP

------------------- ------------------- -------------------

2017-06-19 23:13:18 2017-06-19 23:16:41 2017-06-19:23:13:18

SQL> create or replace view smith.view1 as select a from smith.tab1;

View created.

SQL> select created,last_ddl_time,timestamp from dba_objects where object_name='VIEW1';

CREATED     LAST_DDL_TIME TIMESTAMP

------------------- ------------------- -------------------

2017-06-19 23:13:18 2017-06-19 23:17:34 2017-06-19:23:13:18

SQL> alter table smith.tab1 add b int;

Table altered.

SQL> select created,last_ddl_time,timestamp from dba_objects where object_name='VIEW1';

CREATED     LAST_DDL_TIME TIMESTAMP

------------------- ------------------- -------------------

2017-06-19 23:13:18 2017-06-19 23:17:34 2017-06-19:23:13:18

上面的测试分别执行了重建view，更改view定义但view结果不变，更改view基础表但view结果不变，timestamp都没有变化，可以这样理解：只要对象的结构描述不变，timestamp就不会变（这也加深了我对specification这个单词的理解）。
我们在更改view的基表时，last_ddl_time没有变化，也就是说last_ddl_time记录的是在当前对象ddl语句执行的时间。